Optical space switches are widely required for the routing of optical signals in modern optical networks, particularly in systems operating by wavelength division multiplexing (WDM), and there are various different approaches and materials for realizing them. The material systems typically used in the fabrication of optical space switches include polymers, silica, silicon on silica (SOI), lithium niobate and III-V semiconductor. Polymers and silica based switches using the thermo-optic effect are inherently slower than other types. Devices which operate via the electro-optic effect in lithium niobate or by using electro-optic polymers, suffer from critical stability of their switching characteristics under the applied electric field used to control them. Indium phosphide (InP) based semiconductor is, therefore, a preferred material for the fabrication of optical switches, as it has the advantages of high speed and compatibility with other InP based devices.
Optical switches implemented in a semiconductor planar waveguide structure can be classified into three broad categories, namely the direct coupler, the gain switch and the index switch. The direct coupler type functions by constructive and destructive interference of light propagating between different channels. The gain switch amplifies light in the desired channel and absorbs light in the non-desired channel. Finally, the optical index switch operates by reflecting or focusing light via a local change in the refractive index, which is induced by the injection of electric current or the application of an electric field. One of the most promising candidates among the various types of optical index switch is the digital optical switch (DOS). The state of the art DOS consists of very narrow angled Y-branches or X-crossing with electrodes placed in various configurations on strategic locations at the proximity of the branching point. Various implementations and optimizations of such switches are disclosed in U.S. Pat. No. 6,094,516, U.S. Pat. No. 5,970,186, U.S. Pat. No. 5,537,497 and U.S. Pat. No. 5,991,475.